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Vertical-Cavity Surface-Emitting Lasers for Medical Diagnosis Downloaded from orbit.dtu.dk on: Dec 20, 2017 Vertical-cavity surface-emitting lasers for medical diagnosis Ansbæk, Thor; Yvind, Kresten; Chung, Il-Sug; Larsson, David Publication date: 2012 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Ansbæk, T., Yvind, K., Chung, I-S., & Larsson, D. (2012). Vertical-cavity surface-emitting lasers for medical diagnosis. Kgs. Lyngby: Technical University of Denmark (DTU). 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Vertical-cavity surface-emitting lasers for medical diagnosis PhD Dissertation Submitted to the Department of Photonics Engineering at The Technical University of Denmark in partial fulfillment for the degree of Doctor of Philosophy Thor Ansbæk 2012 Preface This dissertation has been submitted to the Department of Photonics Engineer- ing at The Technical University of Denmark (DTU) for the partial fulfillment of the degree of Doctor of Philosophy (PhD). The main supervisor has been Associate Professor Kresten Yvind. I gratefully acknowledge the PhD Scholar- ship funded by DTU. Part of the thesis work has been funded by the NanoNose project and the support from the Danish Agency for Science and Technology’s Program Commision on Nanoscience Biotechnology and IT (NABIIT) is ac- knowledged. The motivation of the work has been to combine Vertical-Cavity Surface- Emitting Lasers with Micro-Opto-Electro-Mechanical Systems in order to ad- vance the field of Optical Coherence Tomography (OCT). It has been a highly motivating subject to work on and I would like to thank my main supervisor Kresten Yvind for giving me this opportunity and supporting my project in every manner. A cornerstone in the project has been the use of the novel High- index Contrast subwavelength Grating (HCG) and to this aim I would like to thank Il-Sug Chung for his supervision into this for me completely new field. Last but not least I would like to thank David Larsson for his supervision on the Volatile Organic Compound (VOC) sensor. The initial part of the PhD was spent on getting acquainted with Vertical- Cavity Surface-Emitting Laser (VCSEL) processing by the fabrication of bottom emitting VCSELs. Simultaneously, experiments where done coating an off-the- shelf VCSEL with polystyrene and measurering the response to Acetone vapour. I would like to thank Claus Højgård Nielsen for doing the plasma polymeriza- tion of the polystyrene coatings and Søren Dohn for giving me access to his environmetal chamber for performing the experiments. The second part of the PhD project has been spent on the fabrication of a tun- able Fabry-Pérot filter with a HCG top mirror. I would like to thank Jong-Min Kim for his initial work on growing epitaxial wafers with InGaP and AlInP. In order to fabricate subwavelength gratings dry etching was a neccesity and I would like to acknowledge the efforts of Martin Schubert and Jin Liu on the GaAs dry etching using Reactive Ion Etching (RIE) and Inductively Coupled Plasma (ICP), respectively. The aim was to use the tunable filters as part of a Fourier Domain Mode-Locked (FDML) swept source and I would like to thank Sebastian Marschall for discussion on tunable filters for OCT and his help on setting up a swept laser for characterization of the filters. The main part of the characterization of the filters was done using a fiber-based transmission setup and I would like to thank Radu Malureanu for his help and guidance. The last year of the PhD has been a race to develop a tunable VCSEL. I would like to thank Elizaveta Semenova for shedding light on the art of epitax- 2 ial growth. I greatly appreaciated the help of Nadezda Kuznetsova and Sara Ek on the micro photoluminescene setup which was instrumental in measuring non-lasing VCSELs. Thanks go to Martin Schubert for setting up the character- ization setup. Silvan Schmid is thankfully acknowledged for his help with laser Doppler vibrometer measurements on the mechanical properties of the VCSELs. Ole Hansen and Erik V. Thomsen are both thanked for their supervision on the Micro-Electro-Mechanical Systems (MEMS). Having been associated to DTU Fotonik and the adjacent DTU Nanotech for almost 7 years I have had the pleasure to work along a long list of friendly collegues and students. I have greatly appreciated the helpful environment. In particular I would like to thank Troels Suhr Skovgård, Sara Ek, Róz˙aShirazi and Andrei Andryieuski for the cheerfull atmosphere upheld in our office - plastered with cake pictures. Lastly I would like to thank my family for their support - in particular my girl friend Benedicte Ersted Jensen who has supported me all the way and made sure that life went on as usual outside the world of research. This thesis is dedicated to my grandmother Jutta Ursula Elisabeth Chris- tensen. Thor Ansbæk September 30, 2012 Supervisors: Kresten Yvind Il-Sug Chung David Larsson Department of Photonics Engineering Technical University of Denmark Ørsted Plads 344 2800 Kgs. Lyngby Abstract This thesis deals with the design and fabrication of tunable Vertical-Cavity Surface-Emitting Lasers (VCSELs). The focus has been the application of tun- able VCSELs in medical diagnostics, specifically OCT. VCSELs are candidates as light sources for swept-source OCT where their high sweep rate, wide sweep range and high degree of coherence enable deep probing of tissue at acquisition rates that will eliminate the effects of rapid involuntary eye movements. The main achievement of the dissertation work has been the development of an electro-statically tunable VCSEL at 1060 nm with wide tuning range and high tuning rate. The VCSEL is highly single-mode and inherently polarization stable due to the use of a High-index Contrast subwavelength Grating (HCG). HCG VCSELs are presented with 1.5% relative tuning range at a tuning rate of 850 kHz. The thesis reports on the analysis of narrow linewidth Fabry-Pérot filters with dissimilar mirrors and the design of such Fabry-Pérot cavities for VCSELs. Fabrication of InGaAs multiple quantum wells with GaAsP strain balancing layers is covered together with the growth and wet chemical etching of InAlP. The fabrication of the proposed Fabry-Pérot filters and VCSELs is outlined and the results on their characterization reported. 4 Danish Resumé Denne afhandling omhandler design og fremstilling af overfladeemitterende ver- tikal kavitets lasere med variabel resonansbølgelængde. Disse lasere udvikles med henblik på brug indenfor medicinsk diagnostik, nærmere bestemt optisk kohærens tomografi. Denne type laser er oplagt som lyskilde til optisk ko- hærens tomografi hvor den høje repetitionsrate, brede bølgelængde tunbarhed og høje grad af kohærens muliggør dybdeafbildning af øjets lagstruktur uden tab af opløsning på grund af ufrivillige øjenbevægelser. Hovedresultatet i afhandlingen er udviklingen af en overfladeemitterende ver- tikal kavitets laser ved 1060 nm hvor bølgelængden ændres hurtigt i et bredt om- råde ved elektro-statisk kraft. Denne type laser udmærker sig endvidere ved at være monokromatisk og lineært polariseret. Denne polariseringsbestemthed er opnået ved brug af et optisk gitter med en periode mindre end lysets bølgelængde og et brydningsindeks meget højere end det omgivende materiale (luft). Med denne type laser demonstreres en relativ bølgelængdeændring på 1.5% af cen- terbølgelængden ved en repetitionsrate på 850 kHz. I afhandlingen gennemgås teorien for et Fabry-Pérot optisk filter med smal optisk båndbredde hvor filterets to spejle er af forskellig type. Teorien for et Fabry-Pérot filter udvides til at gælde for overfladeemitterende lasere og de relevante designovervejelser for lasere, der kan dække et bredt spektralt om- råde, gennemgås. Afhandlingens andet store resultat er udviklingen af frem- stillingsprocessen til at lave disse Fabry-Pérot filtre og lasere med InGaAs kvan- tebrønde under tøjning. For at kunne danne et tomrum som muliggør ændring af bølgelængden ved elektro-statisk kraft er offeræts af InAlP undersøgt. På baggrund af de valgte designs fremstilles Fabry-Pérot optiske filtre og over- fladeemitterende vertikal kavitets lasere, og deres elektro-optiske egenskaber undersøges eksperimentelt. 5 Contents 1 Introduction 9 1.1 Optical coherence tomography . 9 1.2 Tunable semiconductor lasers . 11 1.3 Stateoftheart ............................ 13 1.4 Thesisoutline............................. 15 2 Theory and design 17 2.1 Fabry-Pérotetalon .......................... 17 2.2 Mirrors ................................ 18 2.2.1 Method ............................ 19 2.2.2 HCG.............................. 20 2.3 Fabry-Pérot filter . 24 2.3.1 Resonance wavelength . 24 2.3.2 Transmission ......................... 26 2.4 HCG-VCSEL ............................. 29 2.4.1 Lasingcondition ....................... 31 2.4.2 Method ............................ 32 2.4.3 Two mirror Fabry-Pérot cavity . 32 2.4.4 Three mirror Fabry-Pérot cavity . 36 2.4.5 Summary ........................... 38 2.5 Electro-static actuation . 40 2.5.1 Static operation . 40 2.5.2 Dynamic operation . 42 3 Device fabrication 46 3.1 Epitaxialgrowth ........................... 46 3.1.1 InGaAsMQW ........................ 48 3.1.2 VCSEL ............................ 50 3.1.3 InAlP ............................. 51 3.2 Sacrificial release etch . 53 3.2.1 General considerations . 53 3.2.2 InAlP sacrificial etch . 54 3.2.3 Results ............................ 55 3.3 Grating pattern transfer . 56 3.3.1 Si3N4 mask.........................
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